1. Technical Field
The invention relates to an optoelectronic sensor and to a method for the measurement of distances or distance changes in accordance with the light transit time principle in accordance with the preamble of claim 1 and claim 12 respectively.
2. Description of Related Art
The distance of objects can be determined in accordance with the known light transit time principle using optoelectronic sensors. For this purpose, in a time of flight process, a short light pulse is transmitted and the time up to the reception of a remission or reflection of the light pulse is measured. Alternatively, in a phase process, transmitted light is amplitude modulated and a phase shift between the transmitted light and the received light is determined, with the phase shift likewise being a measure for the light transit time. Due to eye protection regulations, the last named phase modulation processes are in particular less suitable with low-remitting targets due to the required large integration times. In the pulse process, the integral power can be profitably used such that short pulses can be transmitted at a high energy density and the signal-to-noise-ratio is thus improved for the single shot.
The distance measurement can be necessary, for example, in vehicle safety, in logistics automation or factory automation or in safety engineering. A distance measurement device based on a reflected light beam can in particular respond to a distance change of the reflector or of the reflecting or remitting target. A special use is a reflection light barrier in which the distance between the light transmitter and the reflector is monitored. The light transit time process is also the principle according to which distance measuring laser scanners work whose position vector measures a line or even an area.
If the resolution of the distance measurement ought to reach a precision in the range of a few tens of millimeters, the light transit time must be determined exactly in an order of magnitude of hundreds of picoseconds. To achieve a distance resolution of a millimeter, six picoseconds have to be covered metrologically. Such a precision can only be realized with very cost-intensive electronics with conventional transit time systems.
More cost-effective components such as FPGAs (field programmable gate arrays) and other programmable digital logic components typically have operating frequencies in the range of some hundreds of MHz. Nanoseconds, but not picoseconds, can thus be resolved.
The sampling of the received signal for the determination of the reception time in digital components always needs a discrete time pattern and the time resolution is restricted to that of the time pattern. It is known at the reception side to improve the resolution by interpolation. However, this again requires a high effort for the function fit on which the interpolation is based. In addition, the precision of interpolation is limited.
Transmission times can, in contrast, only be selected with the precision which the electronics make available, that is they are ultimately dependent on the smallest clock time which the control can generate. A precision in the range of picoseconds or even fractions of picoseconds is thus not possible.
It is therefore the object of the invention to provide a possibility for the distance measurement in accordance with the light transit time principle with a higher time precision.